1. Field of the Invention
This invention relates generally to multipurpose semiconductor circuits used, for example, for forming a gain control circuit or switching circuit, and more particularly to solid state multipurpose circuits of the balanced type employing a novel semiconductor device of the three-terminal type with a superior symmetrical characteristic.
2. Description of the Prior Art
Several types of multipurpose semiconductor circuits have been proposed for use as, for example, a gain control circuit or a synchronous detector such as a color demodulator for demodulating a chrominance signal of a composite color television signal. In such circuits, at least one active semiconductor element, for example, a transistor is provided and a first signal which is to be controlled is applied to an input terminal connected to the semiconductor element. The semiconductor element is supplied with a second signal and controlled by the same so as to control the level of the first signal or switch the first signal to conduct intermittently in synchronism with the second signal.
There has also been a multipurpose semiconductor circuit of a different type which has a couple of input terminals and at least one output terminal connected to a circuit portion including active semiconductor elements to derive at the output terminal the multiplied result of first and second signals respectively applied to both input terminals in accordance with the operation of the semiconductor elements.
Such multipurpose switching circuits as described above are usually constructed in the form of a balanced type, that is, a pair of signal transmitting paths including, for example, a semiconductor element such as a transistor, are symmetrically provided with a common output terminal and differentially or alternately operate so as to provide increased efficiency or to avoid a change in the D.C. level of the output signal which tends to occur in response to the operation of the semi-conductor element. Accordingly the circuits usually require a number of semiconductor elements performing the gain controlling, mixing or switching operation and moreover are relatively complicated in structure.